"Astronomers using data from the Sloan
Digital Sky Survey found that the solar system contains about 700,000
asteroids big enough to destroy civilization. That figure is about
one-third the size of earlier estimates, which had put the number at
around two million and the odds of collision at roughly one in 1,500
over a one hundred-year period."

A new survey
revises down the likelihood of a massive asteroid hitting the Earth by
20-30%. We're only due to collide with rocks larger than one kilometre
across roughly once every 600,000 years, it concludes "There was a lot of
error in our previous estimates," says astronomer Alan Harris of the
German space agency, DLR.
"It's all because near-Earth asteroids are somewhat brighter than we
thought".
Tom Clarke, Nature, 14 November 2003